This invention relates to a microprocessor and, more particularly, to an integrated circuit (IC) microprocessor having a programmable logic array.
A microprocessor constitutes the most important portion of a microcomputer, and microprocessor IC chips utilizing MOS type transistors are widely used in many industrial fields.
A prior art standard microprocessor chip consists of a plurality of control function blocks which include a Programmable Logic Array (PLA), a timing and control circuit, an Arithmetic Logic Unit (ALU), a program counter, a data pointer (data memory designation register), an output-only latch circuit, an input/output latch circuit, etc.
Each of input/output latch circuits is equipped with a set-only circuit, whereas each of a program counter, output-only latch circuits and a data pointer is equipped with a reset-only circuit. When an external reset signal is applied to a timing and control circuit, the set and reset circuits described above operate in response to an internal reset signal outputted by the timing and control circuit to initialize an IC microprocessor.
When the microprocessor is in its initial state, all the input/output ports of the input/output latch circuits become a logic "1" or an external data "read" mode, while all the output ports of the output-only latch circuits become a logic "0".
However, with the circuit construction of the prior art microprocessor described above, since it is necessary to provide exclusive reset circuits for each of the program counter, the data pointer, output-only latch circuits, and also exclusive reset circuits for the input/output latch circuits, the circuit construction becomes complicated and the number of the circuit elements increases. Moreover, it has been difficult to readily change the initial conditions of the input/output ports and output ports according to the request of the users.
Furthermore, the aforementioned defects make the chip area larger when the circuit is fabricated of a semiconductor integrated circuit.